3. • The word caisson is derived from the French word “caisse” meaning a box. 3 Schematic Diagram of caisson in historical days

4. • In civil engineering , a caisson is defined as a structure which is sunk through ground or water to exclude water and semi –fluid material during the process of excavation of foundations & which subsequently becomes an integral part of the structure. Definition 4

5. Uses of Caisson • To reach the hard bearing stratum for transferring the load coming on supports for bridge piers and building columns. • To serve as an impervious core wall of earth dams, when placed adjacent to each other. 5

6. Uses of Caisson • To provide an access to a deep shaft or a tunnel. • To provide an enclosure below water level for installing machinery, pump, etc. 6

7. Difference between Cofferdam & Caisson • Cofferdam is a temporary structure while caisson is a part of a permanent structure. • At places where cofferdams can not be dewatered successfully caisson is used. • In cases where, cofferdam becomes uneconomical when plan area is small compared to depth of water. 7

8. Reasons for adoption of caisson: • Depth of water • Nature of soil to be penetrated • Permeability of soil below foundation level 8

10. Suitability of Cast-iron • Suitable for caissons of open well type. • Cost is more compared to R.C.C or Steel. • Unsuitable for pneumatic caisson, because of chances of failure due to compressed air. 10

12. Suitability of steel • It is found to be most suitable material • Usually in the form of a double skin of steel plates • The hollow space is filled with cement concrete 12

13. Suitability of Timber • Required at early stages of construction • Now a days practically not adopted because of its weight & chances of fire 13

14. Classification of Caisson CAISSON Multiple or Monoliths Single Box Caissons (Open at top &Closed at bottom) Wells ( Open at top & bottom) Pneumatic Caissons (Open at bottom &Closed at top) Cylinders 14

16. • Box is open at top & closed at bottom • May be built by R.C.C or steel Box Caisson 16

17. Construction of Box Caisson 1) A level bearing surface is prepared by means of dredging or divers 2) The caisson is constructed on shore, launched and floated to the site of work 3) The box is then filled with suitable material to effect further sinking , if necessary. 4) The top is then sealed & further foundation work is started above the water surface. 17

18. Conditions favorable for construction of Box Caisson • Bed preparation is not required • Velocity of water is slow • Loose soil in bed • Depth of water is @ 6 to 8 m 18

19. Points for Box Caisson • Bottom of box caisson is a part of permanent structure • Successfully used for construction of quay walls in series. 19

21. • A well is open at top as well as bottom • It is provided with a cutting edge at the bottom so as to facilitate sinking • Shape is decided by the requirements of the superstructure, vertical & horizontal forces on well, base of the pier or abutment, cost of sinking, chances of tilting and shifting during execution etc. WELLS 21

22. Advantages of circular well • The ratio of sinking effort to skin friction is maximum, because of min. perimeter • The sinking of well is more uniform as compared to other shapes because the perimeter is equidistant at all points from the centre of the dredge hole. 22

23. Advantages of circular well cont… • The only drawback of a circular well is that it causes more obstruction to the waterway then the bridge pier. • A double D shape is more economical then single circular well • A twin circular well aims at combining the advantages of a circular well and a double D well • If number of wells is very large, a monolith is formed 23

24. Single wells Cutting Edge 1 to 2m Bolts or Rods A single well caisson is used where much sinking is not required or where the material to be sunk through is very soft. If soft material covers the site, it is necessary to remove as much of it as possible before placing the caisson 24

25. WELL Scaffolding Towers Single well –Scaffolding towers 25

26. Staining Rails Sand BagsPlanks Joists Loading of a well 26

27. Procedure adopted for Construction of Single well • Depending on superstructure and structural requirements suitable size & shape of well are determined. • If the caisson is to be sunk through water, it is partially constructed on land, floated and located in its correct position. 27

30. Construction Procedure 1. Suitable site is selected 2. Area of the dry dock should be kept bigger then the monolith 3. Door should be provided with sufficient width to take out monolith. 4. Monolith height should be such that it will remain half submerged. 30

31. 5. Dredging then stars as per the sequence shown in fig. 6.After dredging to required depth, the whole monolith is gradually sunk uniformly up to required level 7. The bottom is sealed by concrete and wells are filled with concrete and cap is provided at the top to finish construction Construction Procedure 31

32. Advantages of monoliths • All the work is done above water & hence reduction in cost. • There is no theoretical limit for the depth up to which a monolith can be sunk • The unskilled labour can be employed for the construction of monoliths 32

33. Disadvantages of monoliths • Foundation level is not available for inspection • The rate of sinking can not be estimated • Monolith may have moderate amount of tilting • Less strength of concrete 33

34. Use of Monoliths • Widely used for foundation work of bridge piers, quay walls etc • Use of monoliths is justified for depth of water which will be unsuitable for the cofferdam or prohibit the use of pneumatic caisson 34

37. Construction procedure • Cylinder is brought to site & kept in vertical position before sinking starts • Cylinders are usually taken in lengths of 2 to 3 m • Metal cylinders …. • After removing the material from inside of the cylinder, the space is filled with concrete for the full height. 37

38. Advantages of Cylinders • Obstruction to the water flow is less • Quantity of excavation is small • Simple construction • Cheap in cost • Do not require expensive and complicated lifting devices 38

39. Disadvantages of cylinders • Difficult to sink them vertically in running water • Load bearing capacity is less due to small dia. • More chances of tilting • Thickness is less, hence not suitable for high bridges • Require additional loading for sinking ( self wt. is less) 39

40. 40

41. Working Chamber Muck Bucket Wall Of Caisson Air for Compressor Hoisting Rope Or cable Man Lock Shaft with ladder or lift Air Lock Compressed Air pipe Water surface SECTION OF PNEUMATIC CAISSON 41

42. Stages of Construction 42

43. Modern Pneumatic Caisson : How it works? 43

44. Compressed Air supply system 44

45. Pneumatic caisson • Open at bottom & closed at top • Compressed air is used to remove water from the working chamber • Adopted due to undulated soil conditions • Maximum depth is limited considering health of workers 45

46. Air pressure • p = wh • Where , p = pressure in KN/m2 w = density of water in KN/m2 = 9.81 KN/m2 h = head of water in m 46

47. Air pressure • The limiting air pressure under which a man can work is @ 343 KN/m2 • So, maximum depth of water up to which a pneumatic caisson can be adopted from health point of view is 343/9.81 = 35 m • The pneumatic caisson can be used for the depths of water ranging from 12 to 35 m. 47

48. Advantages of pneumatic caisson • Inspection of work • Concreting of foundation • Easy removal of obstruction • Checking of verticality of caisson • Reliable quality of work 48

49. Disadvantages of Pneumatic caisson • Extreme care is required to provide suitable health controls for labors. • It requires high degree of skill in operation. • Cost of construction is high. • The depth of penetration is limited below water level. (35m max.) 49

52. Miscellaneous steps to avoid caisson sickness • Use of excessive alcoholic drinks avoided • Hot coffee should be avoided • Use of warm cloths • Enough meal & refreshments should be provided • Workers should be conversant with the use of bells, signals, telephones etc. • No inflammable material should be allowed inside of caisson 52

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